Heretofore, curved glass sheets having various shapes or curvatures have been used as automobile windows. Various kinds of methods have been proposed as the method for producing that sort of curved glass sheets. Among these methods, a method for press-bending in a heating furnace is suited to realize a complicated shape, such as a shape having a vertical section substantially like the letter S, or a deeply bent shape, since this method can bend a glass sheet in a high temperature state. For example, JP-A-1-52628 discloses a technique that a hollow member having a box-like shape is used as a lower mold to put a glass sheet thereon and that air is supplied into the hollow member to raise the pressure in the hollow member to push the entire area of a rear side of the glass sheet against an upper mold by the air pressure. This technique is said to be able to easily realize a complicated shape since it is possible to uniformly pressurize the rear side of the glass sheet in a wide range.
However, since the above-mentioned conventional technique utilizes both press-bending and air-pressuring to instantly bend a substantially flat glass sheet in a desired complicated shape or a desired deeply bent shape, this conventional technique has a problem in that the glass sheet has an optical distortion created in several portions or has a crease formed in a surface. That sort of problem is caused because a substantially flat glass sheet is greatly bent in a short time period by press-bending and air-pressuring. Although that sort of problem may be avoided by providing a sufficient time period for bending, a new problem that the productivity of curved glass sheets lowers is caused in this case.
It is normal that the lower mold and the upper mold utilized for press-bending have bending surfaces covered with woven cloth, such as heat-resistant cloth, in order to prevent a glass sheet from being damaged. In order to provide a glass sheet with a complicated shape or the like, both molds are formed in greatly curved shapes to conform to the shape of a curved glass sheet to be produced. The molds are quite different from the glass sheet in terms of shape before bending. For this reason, the glass sheet is apt to have local portions brought into strong contact with a mold. As a result, there has been caused a problem that the heat-resistant cloth on these local portions is apt to be broken while repeatedly making press-bending of glass sheets.
In some cases, when a glass sheet, which is quite different from the molds in terms of shape, is forcibly pressed, the glass sheet has the impression of a ring for supporting the glass sheet marked on a peripheral portion of the glass sheet.
The present invention is proposed to avoid the problems of the conventional technique. It is an object of the present invention to provide a method and an apparatus for bending a glass sheet, which are capable of producing, with high productivity, a glass sheet having such a complicated shape or a deeply bent shape that is difficult to be realized by the conventional technique, or which are capable of rapidly making preliminary bending.